go.crypto/ssh: fix tests with -cpu 1,2.
go.crypto/ssh: fix tests with -cpu 1,2.

When running the ssh tests several times (e.g. with -cpu 1,2), the
second run would fail because testing globals had been altered. This
change avoids altering the globals since the default worked anyway.

Fixes issue 4715.

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/7903045

cmars/go.crypto,isislovecruft/go.crypto,cmars/go.crypto,isislovecruft/go.crypto,polydawn-ports/go.crypto,dizzyd/go.crypto,xman1101/terminal,tonnerre/golang-go.crypto,xman1101/terminal,tonnerre/golang-go.crypto,dizzyd/go.crypto,drhax9908/go.crypto,polydawn-ports/go.crypto,drhax9908/go.crypto